1. Field of the Invention
This invention relates generally to thermoplastic elastomer (TPE) materials. Thermoplastic elastomers are broadly defined as rubber-like materials that, unlike conventional vulcanized rubbers, can be processed and recycled like thermoplastic materials, yet have properties and performance similar to that of vulcanized rubber at service temperatures. The invention more specifically relates to thermoplastic vulcanizates (TPV), which are thermoplastic elastomers with a cross-linked rubbery phase produced by the process of dynamic vulcanization. The thermoplastic vulcanizates of the invention are foamed materials produced by physical or chemical blowing agents, wherein processing characteristics and foam properties are improved by the inclusion of a modifier derived from polytetrafluoroethylene. The invention also relates to foamed articles obtainable by the process of the invention.
2. Description of the Prior Art
There has been considerable activity on the development of thermoplastic vulcanizate compositions, especially those based on polyolefin thermoplastic resins, which have good foaming properties, and on processes for producing foams having improved properties. U.S. Pat. No. 5,070,111, incorporated herein by reference, discloses a process of foaming thermoplastic elastomer compositions using water as the sole foaming agent. U.S. Pat. Nos. 5,607,629 and 5,788,889, both incorporated herein by reference, describe methods for the production of foamed thermoplastic elastomer profiles by extrusion with a water blowing agent. U.S. Pat. No. 5,824,400 discloses foamed thermoplastic elastomer compositions which incorporate styrenic elastomers. Published European Patent Application No. 0 860 465 teaches a method of foaming thermoplastic elastomers using a water containing chemical compound which releases water at temperatures above the melting point of the thermoplastic elastomer. Published European Patent Application 0 872 516 discloses the use of polypropylene resins having specific rheological properties to enhance the foaming performance of olefinic thermoplastic elastomers.
However, the problems of providing thermoplastic elastomer foams which are soft, with good surface smoothness, low water absorption, improved compression set and compression load deflection, and having fine and uniform cell structure have not been overcome by prior art.
The present invention is based on the discovery that superior thermoplastic elastomer foams can be produced by incorporating into the thermoplastic elastomer, prior to foaming, an acrylic-modified polytetrafluoroethylene. Incorporation of this additive provides a very soft foam product having a number of desirable attributes, including improved processability, high melt strength, high cell density and uniformity, smooth surface, low water absorption, with improved compression set and compression load deflection.
In detail the present invention relates to a process for foaming a thermoplastic elastomer using a physical or chemical blowing agent, wherein an acrylic-modified polytetrafluoroethylene is incorporated into the thermoplastic elastomer composition prior to foaming. Sufficient acrylic-modified polytetrafluoroethylene is incorporated to be effective in achieving the desired attributes. The invention also encompasses thermoplastic elastomer compositions containing the acrylic-modified polytetrafluoroethylene, and foamed articles prepared therefrom.
Thermoplastic Elastomer
Thermoplastic Resin Component
Thermoplastic resins suitable for use in the compositions of the invention include thermoplastic, crystalline polyolefin homopolymers and copolymers. They are desirably prepared from monoolefin monomers having 2 to 7 carbon atoms, such as ethylene, propylene, 1-butene, isobutylene, 1-pentene, 1-hexene, 1-octene, 3-methyl-1-pentene, 4-methyl-1-pentene, 5-methyl-1-hexene, mixtures thereof and copolymers thereof with (meth)acrylates and/or vinyl acetates. Preferred, however, are monomers having 3 to 6 carbon atoms, with propylene being most preferred. As used in the specification and claims the term polypropylene includes homopolymers of propylene as well as reactor and/or random copolymers of propylene which can contain about 1 to about 30 weight percent of ethylene and/or an alpha-olefin comonomer of 4 to 16 carbon atoms, and mixtures thereof. The polypropylene can have different types of molecular structure such as isotactic or syndiotactic, and different degrees of crystallinity including materials with a high percentage of amorphous structure such as the xe2x80x9celasticxe2x80x9d polypropylenes. Further polyolefins which can be used in the invention are high, low, linear-low and very low density polyethylenes, and copolymers of ethylene with (meth)acrylates and/or vinyl acetates.
The polyolefins mentioned above can be made using conventional Ziegler/Natta catalyst systems or by single site catalyst systems. Commercially available polyolefins may be used in the practice of the invention.
The amount of thermoplastic polyolefin resin found to provide useful thermoplastic elastomer compositions is generally from about 8 to about 90 weight percent. Preferably, the thermoplastic polyolefin content will range from about 9 to about 60 percent by weight.
Elastomer Component
Suitable rubbers include non-polar, rubbery copolymers of two or more alpha-monoolefins, preferably copolymerized with at least one polyene, usually a diene. Saturated monoolefin copolymer rubber, for example ethylene-propylene copolymer rubber (EPM) can be used. However, unsaturated monoolefin rubber such as EPDM rubber is more suitable. EPDM is a terpolymer of ethylene, propylene and a non-conjugated diene. Satisfactory non-conjugated dienes include 5-ethylidene-2-norbornene (ENB); 1,4-hexadiene; 5-methylene-2-norbornene (MNB); 1,6-octadiene; 5-methyl-1,4-hexadiene; 3,7-dimethyl-1,6-octadiene; 1,3-cyclopentadiene; 1,4-cyclohexadiene; dicyclopentadiene (DCPD); and vinyl norbornene (VNB).
Butyl rubbers are also useful in the thermoplastic elastomer compositions. As used in the specification and claims, the term butyl rubber includes copolymers of an isoolefin and a conjugated monoolefin, terpolymers of an isoolefin with or without a conjugated monoolefin, divinyl aromatic monomers and the halogenated derivatives of such copolymers and terpolymers. Another suitable copolymer within the scope of the olefin rubber of the present invention is a copolymer of a C4-7 isomonoolefin and a para-alkylstyrene, and preferably a halogenated derivative thereof. The amount of halogen in the copolymer, predominantly in the para-alkylstyrene, is from about 0.1 to about 10 weight percent. A preferred example is the brominated copolymer of isobutylene and para-methylstyrene. Natural rubbers are also olefin rubbers suitable for use in the thermoplastic elastomer composition.
The amount of rubber in the thermoplastic elastomer generally ranges from about 92 to about 10 weight percent. Preferably the olefin rubber content will be in the range of from about 40 to about 91 weight percent.
Additives
The thermoplastic elastomer may optionally contain reinforcing and non-reinforcing fillers, plasticizers, antioxidants, stabilizers, rubber processing oils, extender oils, lubricants, antiblocking agents, antistatic agents, waxes, foaming agents, pigments, flame retardants and other processing aids known in the rubber compounding art. Such additives may comprise up to about 70 weight percent, more preferably up to about 65 weight percent, of the total composition. Fillers and extenders which can be utilized include conventional inorganics such as calcium carbonate, clays, silica, talc, titanium dioxide, carbon black and the like. The rubber processing oils generally are paraffinic, napthenic or aromatic oils derived from petroleum fractions. The oils are selected from those ordinarily used in conjunction with the specific rubber or rubber component present in the composition.
In one particularly preferred embodiment of the invention, the inclusion of an adsorptive inorganic additive has been found to improve the odor properties of the foamed products. The addition of an additive such as magnesium oxide in the range of about 0.1 to about 3 weight percent, preferably about 0.5 to about 2 weight percent, based on the total composition, is effective in eliminating odors.
Processing
The rubber component of the thermoplastic elastomer is generally present as small, i.e. micro size, particles within a continuous thermoplastic resin matrix, although a co-continuous morphology or a phase inversion is also possible depending upon the amount of rubber relative to thermoplastic resin and the degree of vulcanization, if any, of the rubber. Preferably, the rubber is at least partially vulcanized, and most preferably it is fully vulcanized (crosslinked).
The partial or full crosslinking can be achieved by adding an appropriate rubber curative to the blend of thermoplastic olefin polymer and olefin rubber, and vulcanizing the rubber to the desired degree under vulcanizing conditions. It is preferred that the rubber be crosslinked by the process of dynamic vulcanization. As used in the specification and claims, the term dynamic vulcanization means a vulcanization or crosslinking (curing) process wherein the rubber is vulcanized under conditions of shear at a temperature above the melting point of the polyolefin component.
Those of ordinary skill in the art will appreciate the appropriate quantities and types of vulcanizing agents, and the conditions required to achieve the desired vulcanization. Any known crosslinking system can be used, so long as it is suitable under the vulcanization conditions for the elastomer component and it is compatible with the thermoplastic olefin polymer component of the composition. Crosslinking (curing) agents include sulfur, sulfur donors, metal oxides, phenolic resin systems, maleimides, peroxide based systems, hydrosilylation systems, high energy radiation and the like, both with and without accelerators and co-agents.
The terms fully vulcanized or completely vulcanized as used herein mean that the olefin rubber component of the composition has been crosslinked to a state in which the elastomeric properties of the crosslinked rubber are similar to those of the rubber in its conventional vulcanized state, apart from the thermoplastic elastomer composition. The degree of crosslinking (or cure) of the rubber can also be expressed in terms of gel content, crosslink density or amount of uncrosslinked rubber which is extractable by a rubber solvent. All of these descriptions are well known in the art. A typical partially crosslinked composition will have less than about 50 to less than about 15 weight percent of the elastomer extractable by a rubber solvent, while a fully crosslinked composition will have less than about 5 weight percent, and preferably less than about 3 weight percent, of the elastomer extractable by a rubber solvent.
Usually about 5 to about 20 parts by weight of the crosslinking agent or system are used per 100 parts by weight of the rubber component to be vulcanized.
As used herein, the terms thermoplastic elastomer and thermoplastic vulcanizate refer to blends of polyolefinic thermoplastic resin and vulcanized [cured; cross-linked] rubber [elastomer]. Such materials have the characteristic of elasticity, i.e. they are capable of recovering from large deformations quickly and forcibly. One measure of this rubbery behavior is that the material will retract to less than 1.5 times its original length within one minute, after being stretched at room temperature to twice its original length and held for one minute before release (ASTM D1566). Another measure is found in ASTM D412, for the determination of tensile set. The materials are also characterized by high elastic recovery, which refers to the proportion of recovery after deformation and may be quantified as percent recovery after compression. A perfectly elastic material has a recovery of 100% while a perfectly plastic material has no elastic recovery. Yet another measure is found in ASTM D395, for the determination of compression set.
Modified Polytetrafluoroethylene
The composition of the invention includes an acrylic-modified polytetrafluoroethylene (PTFE) component. This component is generally described as a mixture of a polytetrafluoroethylene and alkyl (meth)acrylate having from 5 to 30 carbon atoms. One such blend which is particularly suited for use in the process of the invention is available as Metablen(trademark) A-3000, available from Mitsubishi Rayon Co., Ltd.
The amount of the modified polytetrafluoroethylene component in the composition of the invention generally ranges from about 0.1 to about 4 weight percent, based on the total weight of the composition including the thermoplastic resin component, the rubber component, additives and the modified polytetrafluoroethylene component. The preferred amount of modified polytetrafluoroethylene ranges from about 0.5 to about 2 weight percent, with about 1 to about 2 weight percent being most preferred. Alternatively, the amount of acrylic-modified polytetrafluoroethylene can be expressed in terms of the total weight of thermoplastic resin and modified polytetrafluoroethylene. The preferred amount of modified polytetrafluoroethylene, expressed in this manner, ranges from about 8 to about 30 weight percent with a range of about 15 to about 30 weight percent being most preferred.
In the preparation of thermoplastic elastomers of the invention, the acrylic-modified polytetrafluoroethylene was generally incorporated directly into the thermoplastic elastomer during production of the thermoplastic elastomer so that it was an integral part of the composition. Alternatively, the acrylic-modified polytetrafluoroethylene can be mechanically blended with a preformed thermoplastic elastomer composition, or it can be introduced into the foaming process simultaneously with the thermoplastic elastomer.